It seems that a SETI experiment happened decades before Project Ozma -a pioneering SETI project started in 1960 by Cornell University astronomer Frank Drake (image below), at the National Radio Astronomy Observatory at Green Bank, West Virginia. The object of the experiment was to search for signs of life in distant solar systems through interstellar radio waves.The historians at the blog "Letters of Note" uncovered a telegram sent in 1924 by the Chief of Naval Operations, Edward W. Eberle instructing the United States Navy to listen for radio transmissions from the planet Mars.

The telegram read:

“NAVY DESIRES COOPERATE ASTRONOMERS WHO BELIEVE POSSIBLE THAT MARS MAY ATTEMPT COMMUNICATION BY RADIO WAVES WITH THIS PLANET WHILE THEY ARE NEAR TOGETHER THIS END ALL SHORE RADIO STATIONS WILL ESPECIALLY NOTE AND REPORT ANY ELECTRICAL PHENOMENON UNUSUAL CHARACTER AND WILL COVER AS WIDE BAND FREQUENCIES AS POSSIBLE FROM 2400 AUGUST TWENTY FIRST TO 2400 AUGUST TWENTY FOURTH WITHOUT INTERFERRING WITH TRAFFIC”

During the three day period, during which Mars was in opposition to Earth, nothing but static was detected by Navy radio stations.